EP1837893A1 - Measuring device of an HF-plasma system - Google Patents

Abstract

The device (7) has a filter assembly formed as a band pass filter assembly (14,15) and consists of two filter elements. The filter elements are arranged between the stopper circuits. The two filter elements are formed as low pass filter and high pass filter. The filters are passive filters and one filter is arranged as a attenuator pad (21,31).

Description

The invention relates to a measuring device of an RF plasma system having a decoupling device for decoupling one or more signals related to an RF signal and at least one filter device to which such a signal is supplied.

RF plasma process excitation arrangements include an RF generator that provides RF power to a plasma process. The RF power is typically supplied to the plasma process in a narrow band frequency range, particularly around the industrial frequencies of 13.56 MHz and 27.12 MHz. Measuring devices may be provided to measure the power supplied to the plasma process. For accurate regulation and / or control of the power, it is necessary to accurately detect the power delivered, the current, the voltage, the impedance or even the phase between current and voltage.

There are various ways to determine the power delivered to the load of an RF plasma process excitation system. One frequently used is the measurement by means of a directional coupler. A directional coupler couples signals related to power (forward power P _i and reflected power P _r delivered to the load) into related signals. It is also known to measure voltage and current or the impedance and phase angle between voltage and current.

The decoupled signals are often first converted each time into a voltage signal, which stands in a clear relationship to the decoupled signal, e.g. proportional to the root of the power. Then the voltage signal should be filtered. Low-pass filters are known, consisting of inductors and capacities of simple or higher order. They are used to filter the high-frequency components, which usually occur as disturbances or harmonics in the feedthrough line and should not be measured. A low-pass filter arrangement is always necessary on the basis of the sampling theorem even if the measured signal is to be digitized, e.g. with an A-D converter. In plasma systems, however, low-frequency interference also occurs, which can also lead to measured value errors.

Object of the present invention is to provide a measuring device of an RF plasma system, can be determined with the RF signals, or related physical variables, more accurate.

This object is achieved by a measuring device of the type mentioned above in that the filter assembly is designed as a bandpass filter assembly and comprises a first and a second filter element, between which a Entkoppelglied is arranged. A bandpass filter has the advantage that both low-frequency and high-frequency interference, which can lead to measured value errors, can be eliminated. However, a bandpass filter of conventional type would have a high residual ripple in the passband. This in turn would lead to frequency-dependent measurement deviations. Therefore, according to the invention, a decoupling element is provided between the filter elements and the bandpass filter is composed of a plurality of filter elements, preferably of a series connection of a plurality of filter elements, built up. As a result, the properties of the first filter element can be adjusted independently of the properties of the second filter element and vice versa. Thus, an extremely flat passband low attenuation can be achieved. This means that over a large frequency range, the attenuation is constant, with no or only a slight ripple occurs. The RF signal may be a signal related to RF power, such as an RF current or voltage.

In an advantageous embodiment, the first filter element may be formed as a low-pass filter and the second filter element as a high-pass filter. The low-pass filter can be used to filter the high-frequency parts of the signal describing the RF signal. This can prevent high-frequency components from reaching the decoupling element. The decoupler must then not be designed for high frequencies, which makes the band-pass filter assembly inexpensive and uncritical in terms of oscillation tendencies. By the decoupling member downstream high-pass filter, the remaining low-frequency noise can be filtered out. Preferably, the filter elements are passive filters. Preferably, higher order passive filters with Chebyshev characteristics are used to achieve enough steep slopes to adequately attenuate harmonic harmonics and, at the same time, to assure a passband for a given bandwidth depending on frequency variability and pulse rate.

Advantageously, it can be provided that the cutoff frequencies of the bandpass filter arrangement are adjustable. As a result, the bandpass filter arrangement can be designed for different requirements. By appropriate choice of the cutoff frequencies is the Bandpass filter arrangement for measuring the power of a fixed frequency RF generator, for example, 13.56 MHz, or with correspondingly different selected cutoff frequencies for a frequency variability RF generator or for a pulsed pulsed RF generator. Preferably, the cut-off frequencies for the first and the second filter can be set independently of one another, without the attenuation characteristic being changed.

A fine-tuning when commissioning the power measuring device can be done if the filter elements are tuned. The filter elements can be tuned independently of each other.

It is particularly preferred if an attenuation element is arranged in front of the first filter element or in front of the bandpass filter arrangement. By the attenuator adaptation is broadband improved. The broadband improvement means that the adaptation is also improved for frequency components that should not be measured. This may be desirable because the unwanted frequency components can also lead to reflections and thus to measurement result errors in the case of poor adaptation. The first filter element is designed, for example, as a low-pass filter. It blocks all higher-frequency signals. But as it does not absorb them, it reflects them back to the Attenuator. The higher a damping factor of the attenuator, the fewer of these reflected signals are returned to the decoupling. The fewer signals reflected back to the decoupling element, the lower the measured value deviations. A high damping factor at this point is therefore advantageous. The attenuation also improves the constancy in the passband, that is, the ripple in the passband is lower. In the range of 11.8 MHz to 15.3 MHz, with a 10 dB attenuator, a residual ripple of typically 0.064dB, at least 0.008dB, and at most 0.108dB are achieved.

It makes sense to design the attenuator for system resistance. The system resistance can be chosen arbitrarily. Between decoupling member and attenuator may e.g. be arranged a line. Since high-frequency signals are transmitted here, a characteristic impedance must be taken into account for this line. It then makes sense to choose the system impedance of the attenuator equal to the characteristic impedance of the line. Thus, reflections due to mismatch are largely excluded. A resistance value for shaft and system resistances, which is very common in cables, connections and measuring equipment in HF technology, is 50 ohms and is therefore used to advantage. The attenuator may be of any shape, such as an N or T arrangement of resistors. However, the attenuator can also have a resistance transformation, if this is necessary or desirable for the first filter element or the bandpass filter arrangement. The first filter element itself also has a system resistance. This may differ from the system resistance of the attenuator, if desired. Then, the attenuator may be preferably configured to include a resistance transformation from the system resistance of the attenuator to the system resistance of the filter element.

It is particularly advantageous if the decoupling element has an amplifier. As a result, the attenuation caused by the first filter element in the passband can be compensated again. The amplification factor is preferably arbitrarily adjustable. To ensure a sufficiently large signal to noise ratio, this will Attenuator preferably designed so that the signal level after the attenuator not too small fail. Accordingly, the gain of Entkoppelglieds can be set relatively low, for example between 0 and 6dB.

In one embodiment of the invention, a terminating resistor whose resistance value corresponds to the system resistance of the respective filter element can be provided at the output of the first and / or second filter element. This termination resistor should be as broadband as possible, so not only be effective in the measuring range, but especially in the frequency ranges in which the filter elements cause damping. This additionally increases the adaptation and reduces interference due to reflections.

At the output of the decoupling member, a resistor may preferably be connected in series, which corresponds to the system resistance of the second filter element. This resistance should also be as broadband as possible, i. be constant over a certain frequency range, which is preferably larger than the pass band of the band pass filter array, to suppress reflections due to reflections.

In a particularly preferred embodiment it can be provided that a second decoupling member is arranged downstream of the second filter element. Then the filter element is not burdened by the following wiring and changed its properties. The output of the second decoupling element is preferably the bandpass filtered signal.

In one embodiment, a detector member downstream of the bandpass filter assembly and from the filtered signal may be provided generates a DC signal that has a clear reference to the coupled-out signal, for example, the decoupled forward power, the reflected power or the voltage, the current, the impedance or the phase angle between voltage and current.

Further advantages are obtained if a bandpass filter arrangement is provided for a forward power P _i and for a signal describing the reflected power P _r , particularly preferably one bandpass filter arrangement each having an upstream attenuator, particularly preferably one downstream detector member. As a result, bandpass-filtered signals can be generated for both powers and the power P _L supplied to the load can be determined very accurately.

In one embodiment, a difference image may be provided which determines the difference between P _i and P _r . The difference image is preferably connected downstream of the detector elements.

P _i and / or P _{r can be} decoupled in a particularly simple and cost-effective manner if the decoupling device is designed as a directional coupler. The directional coupler has a feedthrough line, in which the current of the power to be determined flows. The directional coupler furthermore has at least one coupling line for coupling out the forward power and reflected power. However, it can also be advantageously provided for each coupling line for coupling the forward power and for decoupling the reflected power. The characteristic impedance of the coupling lines is advantageously designed for the characteristic impedance of the connected lines. By matching the characteristic impedance reflection can be reduced and there is a particularly good match.

The use of two coupling lines makes it possible to couple P _i and P _r separately. In addition, P _i and P _r can be coupled out with different coupling factors.

A development is characterized in that the coupling lines are terminated at least at one end with a terminating resistor whose resistance value corresponds to the characteristic impedance of the coupling line. The terminator prevents reflections and thus ensures accurate measurement results. Preferably, exclusively the terminating resistor is provided at one end of the coupling lines.

If the terminators are tunable, fine tuning can be done.

The invention also includes an RF plasma system comprising an RF generator providing RF power to a plasma load and a power meter previously described.

Further features and advantages of the invention will become apparent from the following description of embodiments of the invention, with reference to the figures of the drawing, which show details essential to the invention, and from the claims. The individual features can be realized individually for themselves or for several in any combination in a variant of the invention.

Fig. 1

eine schematische Darstellung eines HF-Plasmasystems;

Fig. 2

eine Darstellung einer Messeinrichtung;

Fig. 3

eine Detaildarstellung der Bandpassfilteranordnungen der Messeinrichtung;

Fig. 4

einen Dämpfungsverlauf für eine Bandpassfilteranordnung mit Dämpfungsglied;

Fig. 5

einen Dämpfungsverlauf für eine Bandpassfilteranordnung ohne Dämpfungsglied;

Fig. 6

einen Verlauf des Reflexionsfaktors und des Stehwellenverhältnis einer Bandpassfilteranordnung mit vorgeschaltetem Dämpfungsglied;

Fig. 7

einen Verlauf des Reflexionsfaktors und des Stehwellenverhältnis einer Bandpassfilteranordnung ohne vorgeschaltetes Dämpfungsglied;

Fig. 8

ein Dämpfungsglied in n- Schaltung;

Fig. 9

ein Dämpfungsglied in T-Schaltung;

Fig. 10

ein Filterelement mit Tiefpasseigenschaften;

Fig. 11

ein Filterelement mit Hochpasseigenschaften.

Preferred embodiments of the invention are shown schematically in the drawing and are explained below with reference to the figures of the drawing. It shows:

Fig. 1

a schematic representation of an RF plasma system;

Fig. 2

an illustration of a measuring device;

Fig. 3

a detailed representation of the bandpass filter arrangements of the measuring device;

Fig. 4

an attenuation curve for a bandpass filter arrangement with attenuator;

Fig. 5

an attenuation curve for a bandpass filter arrangement without attenuator;

Fig. 6

a profile of the reflection factor and the VSWR of a bandpass filter arrangement with upstream attenuator;

Fig. 7

a profile of the reflection factor and the VSWR of a band-pass filter arrangement without upstream attenuator;

Fig. 8

an attenuator in n-circuit;

Fig. 9

a T-type attenuator;

Fig. 10

a filter element with low pass characteristics;

Fig. 11

a filter element with high pass properties.

FIG. 1 shows an RF plasma process excitation arrangement 1 of an RF plasma system 100, which is connected to a load 3 via a line 2. The line 4 represents a ground connection. About the line 2 RF power is transmitted to the load 3. The RF plasma process excitation arrangement 1 comprises an RF generator 5, which is connected via a line 6 to a measuring device 7. The measuring device 7 may be a power measuring device, which is thus connected between the RF generator 5 and the load 3. Between RF plasma process excitation arrangement 1 and load 3, an impedance matching, not shown, can be arranged.

The measuring device 7 can also be arranged outside the RF plasma process excitation arrangement 1, but it is advantageous to arrange it as shown in FIG. 1 within the RF plasma process excitation arrangement 1, because it can then be used directly for controlling the HF generator 5.

FIG. 2 shows the measuring device 7, which represents a power measuring device. It comprises a decoupling device 8, which is formed in the embodiment as a directional coupler. The decoupling device 8 has a through-line 9, which is connected both to the line 6 and to the line 2. Parallel to the transmission line 9, a first coupling line 10 and a second coupling line 11 are provided. The coupling line 10 serves to decouple the reflected power P _r or a signal related thereto. The coupling line 11 serves to decouple the forward power P _i coming from the HF generator, or a signal related thereto. The coupling lines 10, 11 each have a terminating resistor 12, 13 at one end, wherein the terminating resistors 12, 13 are designed to be tunable, so that a Resistance value can be adjusted, which corresponds to the characteristic impedance of the output device 8. Both coupling lines 10, 11 are connected to a bandpass filter arrangement 14, 15, in which the signals describing the forward power P _l and the reflected power P _r are filtered. Damping members 21 and 31 are connected in front of the bandpass filter arrangements 14 and 15, and respective detector members 39 and 29 are connected downstream. The power values determined in the bandpass filter arrangements 14, 15 are applied to a difference image 16, where the difference between P _i and P _{r is} determined. The difference represents the power P _L delivered to the load 3. This power value can be used to control and / or regulate the RF generator 5.

In FIG. 3, the bandpass filter arrangements 14, 15 are shown in detail, these being constructed analogously to one another. The signals coming from the decoupling device 8 first pass to an attenuator 21, 31, from where they are guided on first filter elements 22, 32 of the bandpass filter arrangements 14, 15. The attenuators 21, 31 preferably have an input resistance, which advantageously equal to the characteristic impedance of the connecting line between the attenuators 21, 31 and the coupling lines 10 and 11 of the Auskoppelglieds 8. Since these lines and all connections in HF technology are usually made in 50 ohm technology, the input resistance of the attenuators 21, 31 is preferably equal to 50 ohms.

The first filter element 22, 32 has a system resistance. The output of the attenuators 21, 31 is designed for this system resistance, possibly the attenuator 21, 31 performs a resistance transformation. The first filter element 22, 32 is preferably designed as a low-pass filter, and in turn is provided with a Terminating resistor 23, 33 completed, the resistance value preferably also corresponds to the system resistance of the first filter element 22, 32. The first filter elements 22, 32 are arranged downstream of a first decoupling element 24, 34 designed as an amplifier. The amplification factor of the decoupling members 24, 34 may be adjustable. This is followed by a resistor 26, 36, which is tuned to the system resistance of second adjoining, designed as a high-pass filter filter elements 25, 35. The second filter elements 25, 35 have a terminating resistor 27, 37. The resistors 26, 27, 36, 37 preferably correspond to the system resistance of the second filter element 25, 35. The second filter elements 25, 35 are followed by second decoupling members 28, 38, which may also be designed as amplifiers. The use of a second decoupling member 28, 38 has the advantage that the second filter elements 25, 35 are not influenced by circuits connected to the bandpass filter arrangements 14, 15. At the outputs of the second decoupling members 28, 38 are the bandpass filtered signals describing the forward power and reflected power. The second decoupling members 28, 38 are connected downstream of resistors 41 and 51, which correspond to the system resistance of the subsequent circuit. The bandpass filter arrangements 14, 15 accordingly comprise the filter elements 22, 32, 25, 35, the resistors 23, 33, 26, 36, 27, 37, 41 and 51 and the decoupling members 24, 34, 28, 38. The bandpass filter arrangements 14, 15 downstream are detectors 29, 39, in which the corresponding power value can be determined either directly or by conversion. In the difference image 16, the difference of the power values can be determined.

FIG. 4 shows, by way of example, the attenuation profile of the voltage transfer function of a bandpass filter arrangement 14, 15 with an upstream attenuator 21, 31 as a function of time Frequency. Between about 10 and 15 MHz is the Durchiassbereich, wherein in this passband, the curve is very shallow and has little waviness, the damping in this area is thus substantially constant. The attenuation in this area is about -6dB. Outside this range, the attenuation curve has steeply sloping edges with an attenuation of over 40 dB for frequencies greater than 20 MHz and less than 6 MHz. Thanks to the attenuator 21, 31 there is no deterioration of the attenuation at higher frequencies.

In comparison, FIG. 5 shows an attenuation profile of the voltage transfer function of a bandpass filter arrangement 14, 15 without an upstream attenuator 21, 31 as a function of the frequency. Significantly here is the insufficient attenuation at frequencies above 100MHz to detect. In addition, the significantly reduced constancy in the passband can be seen. The attenuation makes the bandpass filter constant in the passband.

FIG. 6 shows the profile of the standing wave ratio (dashed line) and the reflection factor (solid line) of a bandpass filter arrangement 14, 15 with an upstream attenuator 21, 31 as a function of the frequency.

FIG. 7 shows the profile of the standing wave ratio (dashed line) and the reflection factor (solid line) of the same bandpass filter arrangement 14, 15 without an upstream attenuator 21, 31. The dashed line shows the voltage standing wave ratio (VSWR). It should be close to 1 for the best possible input match. Note the increase above 15 MHz in Figure 6 with an upstream attenuator to values of about 1.2, whereas in Figure 7 values of over 200 are achieved. The solid line shows the course of the reflection factor MAG S11 [dB]. It should be as low as possible, advantageously less than 20dB. This, as can be seen in FIG. 6, is reliably achieved with an upstream attenuator of more than 100 MHz, whereas without an upstream attenuator (FIG. 7), a reflection factor close to 0 dB ensures nearly complete reflections above 20 MHz.
8 shows an attenuator 21, 31 in n-form consisting of three resistors R. FIG. 9 shows an attenuator 21, 31 in T-shape consisting of three resistors R.

FIG. 10 shows a passive filter element, consisting of inductances L and capacitances C, with low-pass characteristics.

FIG. 11 shows a passive filter element, consisting of inductances L and capacitances C, with high-pass properties.

Claims (14)

Measuring device (7) of an RF plasma system (100) having a decoupling device (8) for decoupling one or more signals related to an RF signal and at least one filter arrangement to which such a signal is fed, characterized in that the filter arrangement as Bandpass filter assembly (14,15) is formed and a first and a second filter element (22, 32, 25, 35), between which a Entkoppelglied (24, 34) is arranged. Measuring device according to claim 1, characterized in that the first filter element (22, 32) as a low-pass filter and the second filter element (25, 35) is designed as a high-pass filter. Measuring device according to one of the preceding claims, characterized in that both filter elements (22, 32, 25, 35) are passive filters. Measuring device according to one of the preceding claims, characterized in that an attenuator (21, 31) is arranged in front of the first filter element (22, 32). Measuring device according to one of the preceding claims, characterized in that the decoupling member (24, 34) has an amplifier. Measuring device according to one of the preceding claims, characterized in that after the decoupling member (24, 34), a resistor (26, 36) is connected in series, which corresponds to the system resistance of the second filter element (25, 35). Measuring device according to one of the preceding claims, characterized in that a second decoupling member (28, 38) is arranged downstream of the second filter element (25, 35). Measuring device according to one of the preceding claims, characterized in that a detector member (29, 39) is provided, which is connected downstream of the band-pass filter arrangement (14, 15) Measuring device according to one of the preceding claims, characterized in that for a forward power and for the reflected power descriptive signal in each case a bandpass filter arrangement (14, 15), particularly preferably in each case a bandpass filter arrangement (14, 15) each having an upstream attenuator (21, 31), particularly preferably each with a downstream detector member (29, 35), is provided. Measuring device according to one of the preceding claims, characterized in that a differential image (16) is provided, which determines the difference (P _L ) from forward line (P _L ) and reflected power (P _r ). Measuring device according to one of the preceding claims, characterized in that the decoupling device (8) as Directional coupler with at least one coupling line (10, 11) is formed, wherein the characteristic impedance of the coupling line (10, 11) is designed for the characteristic impedance of the connected lines. Measuring device according to claim 11, characterized in that the directional coupler has a passage line (9) and two coupling lines (10, 11). Measuring device according to claim 11 or 12, characterized in that the coupling lines (10, 11) are terminated at least at one end with a terminating resistor (12, 13) whose resistance value corresponds to the characteristic impedance of the coupling line (10, 11). An RF plasma system (100) comprising an RF generator (5) providing RF power to a plasma load (3), characterized by a measuring device (7) according to any one of the preceding claims.

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